What is the process where one species splits into two different species?

The term you are referring to is called speciation. Speciation is the evolutionary process by which new biological species are formed. Species are groups of organisms that are able to interbreed and produce fertile offspring, and that are reproductively isolated from other species. Speciation is a gradual process that typically takes place over long periods of time.

There are several different models of speciation, each of which describes a different way in which new species can arise. The most common model of speciation is called allopatric speciation, which occurs when two populations of the same species become geographically isolated from each other and evolve independently, eventually becoming two distinct species. Allopatric speciation can be caused by various factors, such as the formation of a mountain range, the drying up of a lake, or the separation of land masses by a river or ocean.

Another model of speciation is called sympatric speciation, which occurs when two populations of the same species evolve independently in the same geographic location, without any physical barrier between them. Sympatric speciation can occur due to a variety of mechanisms, such as natural selection acting on different traits within the same population, the evolution of reproductive isolation, or the occurrence of polyploidy, a condition in which an organism has more than two complete sets of chromosomes.

The process of speciation can be a complex one, and the specific mechanisms that lead to the formation of new species are still being studied and debated by scientists.